Hot gas to be discharged from an aircraft results, for example, from the operation of an auxiliary power unit (also referred to as “APU” below) which produces hot gas in the form of exhaust gas at a temperature of up to 730° C. Hot gas produced in an APU should be discharged as cold as possible into the environment in order to keep stresses in the structure and/or the outer skin of the aircraft as a result of local temperature peaks as low as possible. Furthermore, particularly in the case of military aircraft, there is the requirement to make the thermal signature of an aircraft as weak as possible, so that the aircraft cannot be located or identified by means of the thermal signature.
In civilian aircraft, APUs are arranged predominantly in the tail cone. Owing to the aerodynamic flow conditions, however, no special measures are required in the region of the tail cone for the exhaust gas cooling. In a departure from such an arrangement, however, it may be expedient in some aircraft configurations to integrate an APU laterally in the fuselage and, for example, under the wing fuselage fairing. In this installation position, the local flow conditions cause the turbine exhaust gas of the APU emerging in flight to lie on the aircraft outer skin, which is thereby heated up.
For the cooling of APUs themselves, it is known to utilise the so-called jet pump principle, thereby eliminating the need for the employed fan of an oil cooler. With this technical solution, the exhaust gas of the APU flowing out of the tail cone produces, owing to the jet pump effect, a negative pressure which results in air being sucked in from the external environment. The sucked-in air flows through a fireproof bulkhead and from there through the oil cooler, after which it mixes with the exhaust gas and is discharged by the exhaust gas system. An additional side effect of this arrangement is that not only the APU itself is cooled, but also the exhaust gas is cooled. However, this technical solution does not achieve a very effective thermal structure protection. Moreover, the power of the APU is reduced, since only very little cooling effect is achieved particularly when starting up and shutting down the APU, and the APU cannot be operated optimally.
A further technical solution for preventing the heating-up of the structure surface by hot exhaust gases is the use of air guide plates on the outer contour of the aircraft. By means of these, the discharged hot gas is led out of the boundary layer of the flow or mixed in such a manner with the ambient air that the structure surface heats up only to a still tolerable degree. The disadvantage of air guide plates on the outer contour, however, is the increase in drag of the aircraft and thus in the direct operating costs.
DE 31 27 106 A1 discloses an infrared suppressor which can be used in a helicopter for cooling hot exhaust gases produced by a driving engine of the helicopter. The infrared suppressor comprises a cowl which is connected to an engine shroud by means of two flanges. An exhaust gas pipe terminates in a large number of exhaust gas nozzles with exhaust gas openings, through which exhaust gas flows out of the exhaust gas pipe into corresponding exhaust gas conduits surrounded by cooling air ducts. Since the cross-sectional area of the exhaust gas conduits is greater than the cross-sectional area of the associated exhaust gas openings of the exhaust gas nozzles, a negative pressure results when the exhaust gas flows out of the exhaust gas openings. This negative pressure has the effect that cooling air which is supplied through inlet openings in the engine shroud is sucked into the exhaust gas conduits and mixed there with the exhaust gas flowing through the exhaust gas conduits.
WO 03/037715 A1 relates to a passive cooling system for an APU of an aircraft, in which a negative pressure is produced in an exhaust gas duct connected to the APU in order to suck ambient air through an air inlet into an oil cooler, a space surrounding the APU, and the exhaust gas duct.
DE 31 29 305 A1 describes a device for infrared suppression for aircraft, which comprises an exhaust gas duct connected to a gas turbine engine. The exhaust gas duct, the gas outlet end of which is designed in the form of a flower-shaped mixer, opens into an air guiding box which can be supplied with secondary air. The secondary air is sucked in in the vicinity of the flower-shaped mixer and mixed with the exhaust gas flow from the exhaust gas duct.
U.S. Pat. No. 3,921,906, U.S. Pat. No. 3,930,627, U.S. Pat. No. 4,018,046 and U.S. Pat. No. 4,876,851 each disclose infrared suppression systems for use in aircraft, in which hot turbine gas flowing through an exhaust gas duct is mixed with cooling air.
The object of the invention is, accordingly, to reduce or eliminate one or more of the disadvantages outlined. In particular, the object of the invention is to propose a device for cooling hot gas to be discharged from an aircraft, with which device the surrounding outer structure is thermally influenced as little as possible and which does not prevent optimum operation of an APU.